Many known discharge lamps produce light by ionizing a vaporous fill material, such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it. The ionizable fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and high pressure sodium lamps. HID lamps differ from other lamps because their functioning environment requires operation at high temperature and high pressure over a prolonged period of time.
Ceramic discharge chambers for HID lamps have been developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. Such lamps with ceramic discharge chambers have been termed “CMH HID” lamps. Metal halide (e.g., CMH) lamps are widely used because they may have a higher efficiency than incandescent lamps. This is economically and environmentally beneficial.
These lamps, however, may sometimes experience reduced light output over time (typically, expressed as lumen maintenance) due to darkening of the inside of the discharge chamber walls. This darkening is due to tungsten being transported from the tip of the electrode during operation to the inside wall, blocking light. Oxygen within the interior space may aid in cleaning the wall and thus can improve lumen maintenance over the lifetime of the lamp. It has been proposed to introduce a calcium oxide or tungsten oxide dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO99/53523. This has been also achieved in an exemplary embodiment with improved lumen maintenance in U.S. Pat. No. 7,868,553 and US Patent Publication 2010/0013417, each of which has a common assignee as the present disclosure.
During the operation of CMH lamps, oxygen is dispensed within the discharge vessel to facilitate a wall-cleaning cycle, which reduces blackening of the discharge chamber walls and extends the lamp life. In such a process, with the aid of an oxygen additive in the fill, the tungsten (W) electrode material deposited on the CMH discharge vessel wall creates tungsten-oxyhalides with the available free halogen and oxygen. Tungsten-oxyhalides have relatively high vapor pressure at the wall operating temperature of a CMH discharge vessel (˜1300K). The evaporated tungsten-oxyhalides will diffuse back to the arc region of the CMH discharge vessel, dissociate at extremely high temperature (˜5000K) of the arc, and finally, re-deposit elemental tungsten on the electrode surface.
The discharge vessel wall is continuously cleaned due to the fact that the tungsten electrode tip evaporation operates continuously in a closed cycle. Initially, tungsten deposits on the discharge vessel wall, which is then followed by the tungsten being transported back from the wall to the electrode tip with the help of the evaporated tungsten-oxyhalides. Thus, the discharge vessel wall is continuously cleaned. This prevents (or completely reduces) wall blackening and improves luminous flux (lumen) maintenance of, especially, “Ultra” branded CMH lamps.
However, this wall-cleaning tungsten halogen chemical cycle only operates in a rather narrow oxygen concentration range. If the oxygen concentration is too low, the cycle does not operate efficiently. More tungsten is evaporated than the amount of tungsten transported back to the W electrode tip. If the oxygen concentration is too high, then the cycle even erodes the cooler parts of the electrode assembly. Too high of an oxygen concentration may also enhance the K diffusion in the tungsten electrode shank. This creates macroscopic K bubbles, which can lead to breakage of the electrode shank. This situation has been observed in some low watt “Ultra” CMH lamps.
Therefore, typically, the nominal power at which the lamp is designed to function determines the quantity of oxygen required within the interior chamber of the vessel. Generally, too much oxygen in the interior space of a discharge vessel of a lamp lowers initial lumens, but too little available oxygen lowers lumen maintenance.
Because of the narrow oxygen concentration range of the wall-cleaning cycle, it is important to accurately dose oxygen into the discharge vessel. The dosing process must also be compatible with standard manufacturing procedures of CMH lamps.
Despite the superlative lumen maintenance shown by many of the lamps described in the above-noted commonly assigned patent and patent publications, there is generally a desire for even higher efficacy and longer life for CMH lamp. There also remains a desire for a system and method to provide accurate oxygen dosing of “Ultra” type CMH lamps with improved luminous flux maintenance over the useful lamp life. There also remains a desire to provide an effective dose measuring method for accurately measuring an oxygen dose for use with a CMH lamp.
In some of the tungsten oxide dispenser mentioned above, oxygen dosing in “Ultra” CMH lamps is currently implemented by adding tungsten-oxide to metal halide dose pellets. However, due to chemical incompatibility of WOx (x=˜2.87) with some of the metal halide dose components (lanthanum, in particular) and the technical difficulties in pellet preparation, the current metal halide dose pellet supplier is only capable of producing NaI-WOx pellets. Therefore WOx is dosed in NaI-WOx pellets and a rest of the metal halides are dosed in a second type of pellet, for example NaI—TlI—LaI3—CaI2.
Due to this double-dosing process, any error in the dosing process (miss-counting of pellets, non-uniformity of mass, chemical composition or geometry of pellets) causes variations in metal halide molar ratio in the total dose fill. This dosing error can considerably modify the spectrum emitted from the dosing component. High spectral variance of emitted light then leads to color and lumen variations, in particular variations of the correlated color temperature (CCT).